For cell-based power calculation, calculating total power of a system comprising a plurality of cells includes characterizing each cell for power calculation, and using the characteristics of each cell in the system to calculate the total power of the system.
A single-stage digital cell is described as an example below. The power of the cell includes leakage power and dynamic power. The leakage power is characterized when the cell is in steady state. The dynamic power is characterized when the cell is switching, an event in which an input transition to the cell initiated an output transition. The dynamic power includes switching power and internal power. The switching power is caused by charging and subsequent discharging of a loading capacitor at the output of the cell. The loading capacitor is partly contributed by input capacitance of each other cell coupled to the output of the cell. The internal power is power supplied by a supply rail VDD during switching of the cell other than switching power and leakage power. For example, for a complementary metal oxide semiconductor (CMOS) cell, the internal power includes power consumption caused by a short circuit current from a supply rail VDD to ground VSS during switching. Therefore, in some approaches, cell characterization for power calculation includes determining input capacitance of the cell and internal energy consumed by the cell with respect to an input transition at an input of the cell and a loading to the output of the cell.
In some approaches, for characterizing input capacitance and internal energy, integration is performed on an input current transition resulted from an input voltage transition at the input and the supply rail VDD or ground VSS of the cell. The input current transition is integrated from a start time to a stop time identified using the input voltage transition. However, this approach ignores an effect of a capacitor coupling an output of the cell to the input of the cell. The effective capacitance of this capacitor seen at the input of the cell is further increased because the output to the input of the cell exhibits gain. The effective capacitance is called the Miller capacitance. As technology advances, the Miller capacitance has a more significant effect on power of the cell and ignoring such an effect results in inaccuracy in power calculation.
Like reference symbols in the various drawings indicate like elements.